Increased demand for improved fuel economy and emissions control has necessitated the development of emission remediation systems capable of reducing harmful exhaust constituents over a wide range of air/fuel mixtures in both fuel-rich and fuel-lean conditions. Stricter emission regulations have lead to the use of chemical elements that cannot be exposed to the high exhaust temperatures exhibited in the past, e.g., greater than 900° C. Exhaust remediation components used in automotive exhaust environment are increasingly susceptible to failure at temperatures exceeding about 700° C. Temperature sensors are frequently being incorporated into exhaust systems such that measures to reduce exhaust temperatures can be initiated to protect these new emission components. Faster light-off time for these temperature sensors (time to activity) is important for emissions control, as high temperature excursions are at the most frequent levels at startup.
Planar temperature sensors are used in a wide variety of applications across many different disciplines. In automotive applications, the resistance values of the planar temperature sensors are generally greater than or equal to about 200 ohms, which is achieved by creating an elongated narrow ribbon of sensing element material having certain resistance characteristics. Where planar temperature sensors are intended to be used in high temperature environments, i.e., environments where temperatures are often above 500° C., they are traditionally manufactured using extremely precisely controlled thin film depositing and etching techniques. In order to ensure that the elongated sensor trace of the planar temperature sensor has a resistance above about 200 ohms, the length, width, and thickness of the sensing electrode are generally tightly controlled. Although such temperature sensors can be produced with thin film and etching method, it is expensive and troublesome with respect to the extremely precise control generally employed with these techniques.
Additionally, it is noted at temperatures greater than or equal to 600° C., the temperature sensing element material may experience grain growth and/or pore coalescence/nucleation, which can cause changes in the circuit resistance. For example, a temperature sensor failure, which may be referred to as a “hard” sensor failure by those of skill in the art, may occur due to the opening of the resistive circuit as a result of voids (pore) growth in the sensing electrodes. Further, a sensor failure, referred to as a “soft” sensor failure by those of skill in the art, may be observed as resistance drift of the sensing electrodes due to the loss of the resistive circuit cross-sectional area.
Temperature sensors used in automotive exhaust environment are increasingly exposed to alkali metal oxides such as K2O and BaO and acidic oxides such as V2O5, WO3, and MoO3. These materials are particularly troubling to temperature sensors, since the materials can easily migrate through temperature sensor protective coatings and significantly change the resistance of the sensing electrode.
Further, temperature sensors used in automotive exhaust environment, including diesel exhaust, are increasingly exposed to steam, hydrogen, and carbon monoxide. Steam, hydrogen, and carbon monoxide are particularly troubling to temperature sensors since these gas phase materials can migrate through temperature sensor protective coatings and cause vaporization of the sensing electrode, scaling, and de-adhesion of sensing electrodes, and failure of the protective coatings.
It is also noted that sensing electrodes are highly permeable to oxygen. Permeability to oxygen at high temperature can accelerate the coalescence of grain boundaries and the sintering of sensing electrodes. Addition of coatings such as high temperature glasses has enhanced barrier properties to the migration of oxygen. Unfortunately, coatings such as glasses do not inhibit oxygen migration enough to stabilize the electrodes for the length of time desired for durability.
Therefore, what is needed in the art are improved methods of stabilizing a temperature sensing element, and temperature sensors that inhibits grain growth and pore coalescence/nucleation in the sensing element material.